Serial position effect is the reason that recall is better for items at the beginning and the end of the sequence than for the items in the middle of the sequence.
Information at the beginning of a sequence is subject to the primary effect and is likely to be recalled because it already has been placed in long-term memory.
Information at the end of a sequence is subject to the recency effect and has an even higher probability of being recalled because it is still in short-term memory.
The poorer recall of information in the middle of the sequence occurs because the information is no longer in the short-term memory but not yet in the long-term memory.